There have been proposed a number of amorphous silicon films usable as a light accepting layer for electrophotographic photosensitive members.
Along with those amorphous silicon films, there have been proposed various methods for their preparation by means of vacuum evaporation, thermal indiced chemical vapor deposition, plasma chemical vapor deposition technique, reactive sputtering, ion plating and light induced chemical vapor deposition.
Among those methods, the method of thermal induced chemical vapor deposition (hereinafter referred to as "CVD method") had been frequently used in various sectors, but nowadays it is usually not employed because elevated temperature is required and a practically usable deposited film can not be obtained as expected.
On the other hand, the method of plasma chemical vapor deposition (hereinafter referred to as "plasma CVD method") has been generally accepted as being the most preferred and is currently used in industry to manufacture an amorphous silicon film on a commercial basis.
However, for any of the known amorphous silicon films, even if it is obtained by the plasma CVD method, there still remain unsolved problems not only regarding its characteristics, particularly electric and optical characteristics, deterioration resistance and use-environmental characteristics upon repeated use, which are to be adapted to its use as a light accepting layer for electrophotographic photosensitive member, but also for other points such as its homogeneity, reproductibility and mass-productivity.
Now, although the plasma CVD method is widely used nowadays as above mentioned, it is still associated with problems due to the fact that it is practiced under elevated temperature conditions and other problems arise in the process, including the apparatus to be used.
Regarding the former problems, because the plasma CVD method is practiced while maintaining a substrate at elevated temperature, firstly the kind of the substrate to be used is limited to one that does not contain a material such as a heavy metal which migrates and sometimes causes changes in the characteristics of a deposited film to be formed and secondly its thickness is likely to be varied on standing whereby the resulting deposited film lacks uniformity in thickness and in homogeneity of the composition which also can sometimes cause changes in the characteristics of the deposited film.
Regarding the latter problems, for the process, the operation conditions to be employed under the plasma CVD method are much more complicated than the known CVD method, and are extremely difficult to be generalized.
That is, there already exist a number of variations even in the corelated parameters concerning the temperature of a substrate, the amount and the flow rate of gases to be introduced, the pressure degree and the high frequency power for forming a layer, the structure of an electrode, the structure of a reaction chamber, the exhaust rate and the plasma generation system, and besides these parameters, there also exist other kinds of parameters. Under these circumstances, in order to obtain a desirable deposited film product, it is required to choose precise parameters from a great number of varied parameters. And there sometimes occurs a serious problem that because of the precisely chosen parameters, a plasma is apt to be in an unstable state which often imparts unexpected troublesome effects to a deposited film to be formed.
As for the apparatus in which the process using the plasma CVD method is practiced, its structure will eventually become complicated because the parameters to be employed are precisely chosen as above stated, and whenever the scale or the kind of the apparatus to be used is modified or changed, the apparatus must be so structured as to cope with the precisely chosen parameters.
In this regard, even if a desirable deposited film usable as a photoconductive layer for electrophotographic photosensitive member should be fortuitously produced, the film product unavoidably becomes costly for the reasons that a heavy investment is firstly necessitated to set up a particularly appropriate apparatus therefor, there still exist a number of process operation parameters even for such apparatus, the relevant parameters are to be precisely chosen from the existing various parameters for the production of such film, and in accordance with such precisely chosen parameters, the process is to be carefully practiced.
Against this background, there is now an increased demand for provision of such a method that makes it possible to practice the process at lower temperature and at a high film forming rate in a simple apparatus to produce a desirable light accepting layer for an electrophotographic photosensitive member which has good uniformity and has practically applicable characteristics, whereby the product will be low-priced.
Likewise, there is a similar situation with respect to other kinds of light accepting layers for electrophotographic photosensitive member such as nitrided silicon film, carbided silicon film and oxidized silicon film.